1. Field of the Invention
The present invention relates to an electret, and more specifically to an electret polymer comprising compounds other than perfluorinated polymer, the electret polymer is prepared from copolymerizing Vdf and other monomers.
2. Description of the Related Art
Electrets are often applied in microphones, filters, dc and ac motors, radiation detectors, dosimeters, memory storage units, vibration detectors, humidity meters, pressure gauges, electrostatics relays, current generators, voltage generators, electron beam deflectors, and other products. The electret can be manufactured by heating and applying an electric field, a corona discharge, or electron bombardment. A monocharge electret can be manufactured by electron bombardment. The electret is polarized when manufactured by the other two methods. The electret must possess long-term stability and be resistant to humidity and contaminants. Perfluorinated polymers such as PTFE, Teflon AF, and FEP can meet these requirements, but are too expensive and insoluble, resulting in diminished processability, thereby limiting their application. Some organic polymers such as polypropylene, polyethylene, and polycarbonate are cheap, chemically stable, and provide excellent mechanical properties and processability, but with a high charge decay rate when used as an electret, resulting in short lifetime and impracticality for long-term application.
William D. Budinger discloses a binder mixture having PTFT as electret particles, PU, and DMF in U.S. Pat. No. 5,384,337. A matrix of fibers is impregnated with the mixture and cured, whereby the electrets are substantially uniformly distributed throughout the matrix to produce an electrostatic porous material. However, William D. Budinger does not disclose any data regarding surface potential decay, resulting in the performance of the electrostatic porous material remaining unknown.
W. Wirges et al. disclose thermosetting electrets with higher Tg such as benzocyclobutene (BCB) and perfluorocyclobutene in the 10th International Symposium on Electrets. The initial surface potential of a film of either of the electrets is approximately 300V when performing corona discharge under 7 KV. However, the electrets are expensive, with low initial surface potential, and with high potential decay rate even at room temperature.
Isao Sumita discloses poly-3,3-bis(chloromethyl)-oxacyclobutane of 200 μ m thick in U.S. Pat. No. 4,046,704, with initial surface potential approximately 600V when disposed in an electric field of 2000V at 160° C. and then cooled to room temperature. The potential decay of the film 30 days from polarization is not apparent but convincing data are again not provided. The initial surface potential of the film is also unclear.
Marvin E. Jones et al. disclose a fluorinated electret in U.S. Pat. No. 6,432,115, such as polyethylene, polypropylene, or polystyrene performing surface fluorination by corona discharge under fluorine containing gas such as C5F12, C2F6, CF4, or NF3 to suppress the absorption of oily aerogels from the environment, resulting in improving filtering effect and lifetime of a filter using the electret. The initial surface potential and potential decay rate of the electret are not disclosed and remained unknown.